According to CTIA, 41% of all U.S. households are wireless-only. In a simple random sample of 300 households, 143 were wireless-only. Is this result unusual?

Approach Because np(1 - p) = 300(0.41)(1 - 0.41) = 72.57 ≥ 10, the binomial probability distribution is approximately bell shaped. Therefore, we can use the Empirical Rule: If the observation is less than μ – 2σ or greater than μ + 2σ, it is unusual.